Under normal operating conditions, electrical transmission and distribution equipment is subject to voltages within a fairly narrow range. Due to lightning strikes, switching surges or other system disturbances, portions of the electric system may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. Left unprotected, critical and costly equipment such as transformers, switching apparatus, and electrical machinery may be damaged or destroyed by such overvoltages and the resultant current surges. Accordingly, it is routine practice within the electrical industry to protect such apparatus from dangerous overvoltages through the use of surge arresters.
A surge arrester is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert the overvoltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When caused to operate, a surge arrester forms a current path to ground having a very low impedance relative to the impedance of the equipment that it is protecting. In this way, current surges which would otherwise be conducted through the equipment are instead diverted through the arrester to ground. Once the transient condition has passed, the arrester must operate to open the recently-formed current path to ground and again isolate or "reseal" the distribution or transmission circuit in order to prevent the nontransient current of the system frequency from "following" the surge current to ground, such system frequency current being known as "power follow current." If the arrester did not have this ability to interrupt the flow of power follow current, the arrester would operate as a short circuit to ground, forcing protective relays and circuit breaker devices to open or isolate the now-shorted circuit from the electrical distribution system, thus causing inconvenient and costly outages.
Distribution transformers convert primary, high voltage levels, such as 2.4 to 34.5 KV, to secondary, low voltage levels, low voltage typically being defined as 1200 volts and less. Distribution transformers include primary and secondary windings which are enclosed in a protective metallic housing. A typical, secondary side voltage level for distribution transformers is 120 volts. Dual secondary side voltages such as 120/240 volts or 240/480 volts are also typical. A dual secondary voltage, such as 120/240 volts, is achieved by constructing the transformer secondary winding in two halves or sections. One end of each of the two winding sections is electrically joined at a predetermined point and typically grounded at this point of interconnection. In this configuration, when the transformer is energized, the voltage between the grounded interconnection point and each line potential terminal will be the same, i.e., 120 volts, and will be equal to one half the voltage between the two ungrounded ends, i.e., 240 volts.
The primary or high voltage terminals of single phase distribution transformers are conventionally designated as the H.sub.1 and H.sub.2 bushings. The low voltage or secondary side line-potential terminals for these single phase transformers are designated as X.sub.1 and X.sub.2, while the low voltage grounded neutral bushing or terminal is designated as X.sub.0. For three-phase transformers, the primary terminals are conventionally referred to as the H.sub.1, H.sub.2 and H.sub.3 bushings, while the secondary line-potential terminals are designated as X.sub.1, X.sub.2 and X.sub.3. The neutral bushing on transformers employing a grounded neutral is usually designated X.sub.0.
The majority of distribution transformers are designed for pole mounting; however, some are built for pad or platform mounting. Regardless of mounting type, distribution transformers are susceptible to damage from lightning induced surges entering their windings. When a lightning surge occurs, the voltage appearing across the primary winding may exceed the insulation strength of the winding, resulting in a flash-over across or through the winding insulation, thereby causing the transformer to fail. It has been conventional practice to provide overvoltage protection for distribution transformers by means of surge arresters applied to the primary, high voltage winding. More specifically, in the case of single phase distribution transformers in which both primary bushings H.sub.1 and H.sub.2 are at line potential, surge arresters have typically been connected between H.sub.1 and ground and between H.sub.2 and ground. In applications in which primary bushing H.sub.1 is at line potential and H.sub.2 is grounded, it is common to connect a single surge arrester between H.sub.1 and grounded H.sub.2. The surge arrester's function is to provide a path by which lightning induced current is diverted to ground, thus preventing flashover of the transformer's winding insulation.
Investigations have been made in recent years concerning lightning induced failures of common designs of overhead and pad mounted distribution transformers. These investigations revealed that despite the presence of state-of-the-art primary-side lightning protection as described above, many such transformer failures are attributable to lightning induced surges entering the transformer via the normally unprotected low voltage terminals, causing failure of the high voltage winding due to the induced voltages. While lightning induced currents entering the low voltage bushings are normally non-destructive, current surges over 5,000 amps are not uncommon. Secondary surges in the order of 3,000 amps can result in potentially destructive induced voltages in the primary winding which may cause the transformer to fail. Thus, it has been determined that primary side arrester protection of the high voltage winding is ineffective in preventing transformer damage due to lightning induced surge currents injected in the secondary windings.
In an effort to protect distribution transformers from such secondary-side surges, various schemes have been employed. First, constructing the transformers with interlaced secondary windings provides good protection from three-wire surges; however, two of the most common types of secondary surges result in two-wire surge injection and interlaced windings offer no protection from such surges. Further, transformers having interlaced windings also are more expensive than those with non-interlaced windings.
Alternatively, or additionally, extra primary winding insulation may be added to provide some protection from both two and three-wire surge injection. This technique is relatively expensive, however, and does not prevent surges from entering the transformer, but merely serves to raise the damage threshold level of the transformer.
Recently, surge arresters of the metal oxide varistor (MOV) type have been applied between the secondary-side phase terminals, X.sub.1, X.sub.2 and X.sub.3, and the grounded neutral terminal, X.sub.0. MOV disks are variable resistors which provide either a high or a low impedance current path through the disk's body depending on the voltage that appears across the MOV disk. More specifically, at the power system's steady state or normal operating voltage, the MOV disk has a relatively high impedance. As the applied voltage is increased, gradually or abruptly, the impedance of the MOV disk progressively decreases until the voltage appearing across the disk reaches the disk's "breakdown" or "turn-on" voltage, at which point the disk's impedance dramatically decreases and the disk becomes highly conductive. Accordingly, if the arrester is subjected to an abnormally high transient overvoltage, such as may result from a lightning strike or power frequency overvoltage, the MOV disk becomes highly conductive and serves to conduct the resulting transient current to ground. As the transient overvoltage and resultant current dissipate, the MOV disk's impedance once again increases, restoring the arrester and the electrical system to their normal, steady state condition.
MOV type secondary surge arresters have been shown to provide adequate two and three wire surge protection for low energy surges of, for example, 10,000 amps or less. Some manufacturers of such arresters claim their arresters are capable of safely dissipating surges of 20,000 amps. However, to date, such MOV secondary arresters have not had the even higher energy discharge capability desirable.
Further, state-of-the-art MOV secondary surge arresters are expensive to manufacture. For example, U.S. Pat. No. 4,809,124 assigned to General Electric Company, describes a high energy, low voltage surge arrester employing MOV disks having a thickness of 0.115 inches and a diameter of 3 inches. With these and even thicker MOV disks, precise and expensive machining is required to provide a relatively flat and uniform contact surface on the MOV disks. Adding to the manufacturing expense is the fact that the MOV disks typically require that an insulative collar be attached around the circumference of the disk to prevent flashover from one facing surface of the disk to the other.
Additionally, some type of housing has traditionally been required to house and support the MOV disks, electrodes and other components which comprise the arrester. Furthermore, to insure consistent and predictable operation of the arrester, a spring, typically a coil spring or a bellville-type washer, has been required within the arrester to impart the force that is required to maintain good electrical contact between the MOV disks, electrodes and other internal components. Besides providing protection and support, the arrester housing has traditionally also been required in order to provide the reactive force necessary for the spring to function for its intended purpose. Although not specifically directed to low voltage, secondary applications, U.S. Pat. No. 4,240,124 assigned to Kearney-National, Inc. depicts in FIG. 1 a typical spring/housing configuration designed to impart an axial force on a stack of varistors. The requirements dictated by the spring and housing complicate the manufacturing and assembly process and lead to additional costs.
Accordingly, there remains a need in the industry for a low voltage surge arrester capable of protecting a distribution transformer from damage or destruction caused by surge currents that are injected into the secondary windings. Preferably, such an arrester would be of the MOV type and would be durable, rugged and be of a low cost construction. Preferably, the MOV disks would be collarless and would not require extensive machining after the disks are fired. The arrester should be suitable for installation in under-oil applications, such as within the transformer tank or enclosure. Preferably, such an arrester would be effective against high magnitude, short duration surges of 40,000 amps or more. Ideally, the arrester would not require a housing to insulate or protect the arrester components or to provide a reaction force for an internal spring. In fact, it would be preferable if such an arrester could be manufactured with a reliable means for maintaining electrical connection between the MOV disks, electrodes and other internal components which did not depend upon a conventional spring and housing to supply the compressive force necessary.
Other objects and advantages of the present invention will become apparent from the following description.